I. Field of the Invention
The present invention relates to a three-dimensional measuring apparatus for optically measuring with high precision the surface shape of a general free curved surface or a lens or mirror having an aspherical surface without contacting the surface to be measured and, more particularly, to an optical measuring apparatus for focusing a laser beam through an objective lens on an object surface to be measured and for detecting a Doppler shift in the frequency of the reflected laser beam which is caused by displacement of the object surface to be measured.
II. Description of the Prior Art
Laser interferometric measuring equipment using an optical heterodyne method is commercially available from Hewlett-Packard (Model HP5526A). At present, this model is known as the most precise measuring instrument which can be easily handled. In addition, this model can be installed on a three-dimensional carrier to constitute a three-dimensional measuring system or a highly precise lathe system.
In conventional optical measuring apparatuses, a corner-cube prism and a mirror are mounted on the carrier, and only the movement of the carrier is measured by a laser interferometer. In the case of the conventional three-dimensional measuring instrument, a measuring probe is shifted relative to the carrier along a surface of the object to be measured. In this case, both a contact type probe and a noncontact type probe are available. However, both can provide a measuring precision of only 1/10 of that in the laser interferometer.